1. Field of Invention
The present invention relates to an improvement in natural circulation type evaporative cooling electric power cable lines.
2. Discussion of the Prior Art
Natural circulation type evaporative cooling power cables are subjected to cooling by taking advantage of the latent heat of vaporization of a liquefiable cooling medium. Such electric power cables have a cooling medium passage therewithin and a cooling unit installed at a position which is higher than or above the cooling medium feeding inlet for the electric power cable. The cooling unit consists primarily of a condenser and a cooling medium reservoir, and strictly speaking, is installed at such a position that the surface of the cooling medium in the reservoir is located at a higher level than the cooling medium feeding inlet. The cooling unit is connected with the cable by means of a feeding pipe and return pipe for the cooling medium to construct a sealed circulation system for the cooling medium in which a suitable quantity of cooling medium is sealed. Such cooling systems make it possible to transmit large amounts of electric power as the cooling medium absorbs heat generated in the cable and vaporizes, and then circulates in the cable naturally when a large load current flows in the cable. Such a system is highly dependable because it does not require any rotary or other positive displacement mechanism for forcing circulation of the cooling medium, such as a pump. Present day cable lines such as shown in U.S. Pat. No. 3,962,529 have return piping which connects the cooling medium outlet of the cooled cable with the cooling unit in such a manner to make it easy for the vaporized cooling medium, such as sold under the trademark Freon, to rise in the return pipe to the cooling unit.
In present day systems wherein the cooling medium outlet of the cooled cable is located at a cable joint box, it is comparatively easy in theory to install the return pipe directly upward. In designing the piping for the actual installation, however, it is sometimes found desirable to position the return pipe at a position which is lower than the cooling medium outlet. Such a piping arrangement yields an even more serious problem in situations wherein the sealing end becomes the cooling medium outlet. In other words, if it is desired to install the cooling unit at a distance from the sealing end, there is a drawback in that it is considerably more difficult to install the return pipe at desired altitudes.
Furthermore, there are also situations wherein the conditions of the land make it difficult to install supporting steel structure for the return pipes and cooling unit. The present invention enlarges the scope of practical application of such cable lines by eliminating restrictive conditions for the piping of return pipes of natural circulation type evaporative cooling power cable lines. More specifically speaking, it makes such supporting steel structure unnecessary, makes it possible to freely select the location of the cooling unit to be installed, and makes it possible to bury the return pipe under the ground.
With natural circulation type evaporative cooling cable lines heretofore in use, dependable circulation was made possible by utilizing the phenomenon wherein the specific gravity of the vaporized cooling medium is smaller than the specific gravity of the cooling medium in liquid phase. However, if the return pipe is installed downward from or below the cooling medium outlet, it is then necessary to force the vaporized cooling medium to move downward. As this is against natural phenomenon, circulation will not be attained. In consequence, the construction of the piping in this manner was impracticable, and this caused a restrictive condition in designing the cable line. By analyzing interesting phenomena discovered in the course of researches, the present invention was discovered, as it was found out that 100% freedom of return piping, which was considered impossible in the past, could be insured by adding a simple mechanism.